1. Field of the Invention
The present invention relates to a print head for a printer and, more specifically, to a dot print head comprising styli each attached to the nose of an armature.
2. Description of the Related Art
Various dot print heads of such type have been proposed. FIGS. 6 and 7 illustrate respective dot print units of first and second conventional dot print heads, by way of example.
Referring to FIG. 6, in the first conventional dot print head, a permanent magnet 2, a support plate 3 and a spacer 4 are placed one over another in that order on one end of a base plate 1. A spring plate 7, a yoke 8 and a guide member 5 are placed one over another on the spacer 4 in a cantilever fashion, and are held in place with a clamping member 6. An armature 12 is attached to the flexible free end of the spring plate 7 adjacent to the yoke 8 and opposite to the core 9 of an electromagnet 11. A stylus 13 is attached to the nose of the armature 12, and is guided by the guide member 5 for movement relative to a platen. The spacer 4, the yoke 8, the armature 12, the core 9 and the base plate 1 form a magnetic path for the magnetic flux of the permanent magnet 2. Normally, the armature 12 and the spring plate 7 are attracted to and biased toward the core 9 by the magnetism of the permanent magnet 2.
When the electromagnet 11 is energized to produce a magnetic force acting opposite to and exceeding that of the permanent magnet 2, the spring plate 7 and the armature 12 are released from the core 9, to allow the stylus 13 attached to the nose of the armature 12 to project from the guide member 5 and to press an ink ribbon and a recording medium, not shown, against the platen for printing.
FIG. 7 illustrates the constitution of the essential portion of a second conventional dot print head disclosed in Japanese Utility Model Laid-open No. 60-3042. The second conventional dot print head is substantially the same as the first conventional dot print head in constitution, except that the rear portion of an armature 12 is longer than that of the armature of the first related dot print head, and a fixed point C of primary vibration of the armature 12 between a restrained position indicated by continuous lines and a free position indicated by broken lines where an impact is applied to a stylus 13 during a printing operation coincides with the instantaneous center of rotation of the armature 12.
As is obvious from FIG. 8 showing the motion of the free end of the armature 12 of the first conventional dot print head, the free end of the armature makes vibrations of higher degree immediately after a printing motion. Therefore, the armature is liable to break, both the contact time and the stabilizing time are long, and the motion of the armature is not converted into printing force efficiently.
As shown in FIG. 9, the armature of the second conventional dot print head does not make vibrations of higher degree and hence this armature does not have the drawbacks of the foregoing armature. However, the high speed of the armature in the return stroke is liable to cause the armature to rebound after the armature is attracted to the electromagnet. If the attraction is increased to suppress the rebound of the armature, the time lag between printing motions is increased and, consequently, driving time is increased, thus reducing printing efficiency.